Defenders:
Fullbacks: Antonio Valencia and Luke Shaw
As the club captain, Valencia is almost guaranteed to start the majority of United's games this season. If not for that, maybe because his closest challenger Matteo Darmian has struggled to get a look in by the manager for a while now and the next in line is a young fullback Diogo Dalot, a new arrival who still lacks the experience to start for Manchester United on a consistent basis.
Luke Shaw, on the other hand, means business this season. He has looked fitter and ready to fight for his spot in the starting eleven. Maybe the sting of being left out of the World Cup squad has got to him, in a good way. If he continues with the momentum he has been building over the last few games then, Ashley Young will struggle to get a look in into the starting lineup for the Red Devils this season.
Center Halves: Chris Smalling and Eric Bailly
Chris Smalling has not been any fan's cup of tea since he joined the club, but you can't argue that he has been the most consistent central defender at United since Jose Mourinho took over. If Mourinho is to build a strong partnership at the back he needs to trust Smalling more often than the other choices he has. If you look at the Brighton game, Manchester United would have done with his ability in the air to deal with Brighton's assaults. Though he's not free from an error here and there, he still represents the best prospect that United have.
Touted as the successor to the likes Nemanja Vidic and Rio Ferdinand, Bailly was seen by most United fans as the defender the club has been craving for ever since the legendary dup departed. But that hasn't happened yet. Bailly appears to be rash and careless, every time he tackles a player near or around the box, one feels like a red card or a penalty will be the result. Having a steady partner like Smalling by his side will help him focus on his strengths and will have a calming influence on the Ivorian.